RESPONSIVE LIFE OF ORGANISMS 471 



On the other hand a stentor will soon cease altogether to 

 react to light touches rapidly repeated. A hydra also will 

 react to touch by contracting its body, and if allowed time 

 between stimulations for complete extension of its length, it 

 will contract again when touched, every time; but if the 

 touch is repeated before a full extension of the body, the 

 animal will soon cease to notice it altogether. In such an 

 organism the stimuli must follow each other in quick suc- 

 cession if they are to modify action. Certain spiders 

 habitually drop from their webs to the ground when dis- 

 turbed. This is their avoidance reaction, and it doubtless 

 carries them often out of an exposed situation into one of 

 less danger, and at times enables them to escape capture by 

 enemies. A certain spider will respond, when a large tun- 

 ing fork is struck near at hand, by dropping from its web. 

 But it will cease to do this (and to put itself to the trouble 

 of climbing up again) after the experiment has been repeat- 

 ed half a dozen times. 



Moreover, the spider unlike the hydra and the stentor and 

 many others of the lower animals, may after daily repetition 

 of this experience become used to the stimulus of the tuning 

 fork and cease to react to it at all after a considerable time. 

 This would seem to indicate growth in power of discrimina- 

 tion between stimuli; for only an animal with some such 

 power could afford to suspend the reaction by means of 

 which it escapes its ordinary enemies. 



Cessation of response to continued but harmless stimula- 

 tion is a wide spread phenomenon in the reactions of animals. 

 It is tooofamiliar for much notice in ourselves. We are not 

 long conscious of contact with our clothing after it is put on, 

 notwithstanding that by contact we get it adjusted properly. 

 And even admonitions of the worthiest sort, too oft re- 

 peated, we let "go in at one ear and out at the other;" that 

 is, they go unheeded. The better the organization of the 



